Lubricant oils have been used to lubricate internal combustion engines, devices in driving systems (e.g., automatic transmissions, shock absorbers and power steerings) and gears having sliding mechanical members for their smooth operation. In particular, lubricant oil for internal combustion engines are used mainly for piston rings, cylinder liners, bearings for crank shafts and connecting rods, valve trains including cams and valve lifters, and other sliding members. They are also used for cooling the engines, cleaning and dispersing combustion products, and prevention of rust and corrosion, in addition to the lubricating purposes. As described above, lubricant oils for internal combustion engines are required to exhibit a variety of functions. These requirements are becoming even more severe, as the engines become more functional, produce higher power and are operated under more severe conditions.
In order to satisfy these requirements, lubricant oils for internal combustion engines are incorporated with a variety of additives, such as antiwear agent, metallic detergent, ashless dispersant and antioxidant. The essential functions of a lubricant oil for internal combustion engines are to prevent wear and seizure by helping the engine operate smoothly under all conditions. Hydrodynamic lubrication prevails in lubricated engine members, but boundary lubrication tends to occur in some sections, e.g., valve trains and dead centers in the cylinders. In general, zinc dithiophosphate or the like is added to prevent wear in the boundary lubrication areas.
More recently, reduction in quantities of spent automobile lubricant oils has been increasingly attracting attention for environmental preservation. In particular, Association of European Automobile Manufacturers is including serviceability of lubricant oils in the specifications of lubricant oils for internal combustion engines. Improved serviceability of the lubricant oils is being required also in Japan.
Air pollution by exhaust gases (in particular, NOx) from diesel engines is becoming severer worldwide, and there are movements to introduce more stringent regulations on NOx and particulate matter emissions from diesel engines. Engine makers are responding to these regulations by an EGR system, which is already adopted for gasoline engines, to clear the NOx regulations. Some of the problems involved in use of an EGR system are still increased quantities of soot in the lubricant oil to aggravate wear of valve trains and piston-cylinder interfaces by soot, making it more difficult to improve serviceability of the lubricant oil.
Moreover, it should be noted that abatement of NOx and particulate matter run counter to each other, when an EGR system is adopted for NOx abatement. One of the methods trying to solve problems of increased particulate matter in an EGR-equipped engine is use of high-pressure fuel injection, where high-pressure fuel is stored in a pressure-accumulating piping system (referred to as common rail) by means of a fuel supply pump and then injected into each engine cylinder under pressure from the common rail via a valve, to improve combustion conditions. It is considered to be essential that the future diesel engine must be equipped with an EGR system and pressure-accumulating type fuel injector simultaneously to clear the more stringent exhaust gas regulations.
It is difficult for the current techniques for preventing wear of valve trains and piston-cylinder interfaces by soot to drastically solve the problems of improving lubricant oil serviceability.
A variety of techniques have been proposed to prevent wear of internal combustion engines, e.g., incorporation of 4 types of additives including zinc dithiophosphate (Japanese Laid-open Patent Application No. 54-103404), a combination of organomolybdenum compound and zinc dithiophosphate (Japanese Laid-open Patent Application No. 54-113604), a combination of organomolybdenum compound, salicylate and bis type succinimide (Japanese Laid-open Patent Application No. 5-230485). The techniques trying to improve serviceability include a combination of organomolybdenum compound, zinc dithiophosphate and polysulfide (Japanese Laid-open Patent Application No. 8-73878, EP699739)
However, in a diesel engine, unlike gasoline engine, the engine oil tends to be contaminated with large quantities of soot evolving as a result of incomplete combustion of diesel fuel oil. It is reported that the soot, having surface activity, may adsorb a polar additive in the engine oil and scrape a coating film away from the friction surface. The required functions of an antiwear agent for diesel engines, therefore, should be much different from those of the agent for gasoline engines under the severe friction conditions with the engine oil contaminated with soot. Therefore, the conventional techniques for compounding antiwear agents, e.g., use of zinc dithiophosphate, may not exhibit sufficient effect of preventing wear under the lubricating conditions with large quantities of soot in the oil. A combination of molybdenum dialkyl dithio-phosphate (Mo content: 200 to 400 ppm), zinc primary alkyl dithiophosphate and salicylate (Japanese Laid-open Patent Application No. 7-207290) and combination of specific contents of sulfurized oxymolybdenum dithiocarbamate and zinc dialkyl dithiophosphate, are some of few additives proposed so far to improve wear-preventive characteristics of the lubricant oils for diesel engines operating under the above conditions.
However, these lubricant oils may not exhibit sufficient effects of preventing wear in an EGR-equipped diesel engine, i.e., under the lubricating conditions with large quantities of soot in the oil. Therefore, the technological development has been strongly demanded for diesel engine lubricant oil compositions which can prevent wear of valve trains and piston-cylinder interfaces.